1. Field of the Invention
The present invention relates to a technical field of processors and, more particularly, to an instruction length determination device and method using concatenate bits to determine an instruction length in a multi-mode processor.
2. Description of Related Art
A multi-mode processor switches between 2N-bit and N-bit instruction modes in order to reduce the space required for storing programming codes typically.
In U.S. Pat. No. 7,149,879 granted to Liang for a “Processor and method of automatic instruction mode switching between n-bit and 2n-bit instructions by using parity check”, the parity is employed to perform the switching. Each N-bit word contains two parts of P-bit parity and (N-P)-bit instruction. When a fetched 2N-bit word is of an even parity, the fetched 2N-bit word is regarded as two (N-P)-bit instructions. Conversely, when the fetched 2N-bit word is an odd parity, it is regarded as a single 2(N-P)-bit instruction.
In U.S. Pat. No. 5,758,115 granted to Nevill for an “Interoperability with multiple instruction sets”, a T bit from a program counter (PC) is employed to determine an X-bit or Y-bit instruction mode performed by the processor, and an instruction of ‘Branch’ is used to switch the value of the T bit from the PC. in U.S. Pat. No. 6,209,079 granted to Otani, et al. for a “Processor for executing instruction codes of two different lengths and device for inputting the instruction codes”, a 2N-bit word fetched by special bit recognition is two N-bit instructions or one 2N-bit instruction.
However, the prior art cannot prolong the length of data once the data structure is determined. The data for storing is necessarily aligned with a same length for easy recognition and the different lengths of data in interlacing is not allowable. Further, the density of data storage cannot be increased, and the complexity of compiler is increased. Thus, the code storage space cannot be optimized.
Therefore, it is desirable to provide an improved device and method to mitigate and/or obviate the aforementioned problems.